1. Field of the Invention
The present invention relates to a method and an apparatus for transmitting and receiving a feedback signal in a cellular mobile communication system having a plurality of base stations. More particularly, the present invention relates to a method of efficiently transmitting and receiving feedback in a Cooperative Multi-Point (CoMP) system supporting downlink transmission of a User Equipment (UE) by cooperation with the base stations, and an apparatus thereof.
2. Description of the Related Art
A mobile communication system according to the related art provides a voice-oriented service. However, the mobile communication system has evolved to support high speed and high quality wireless packet data communication capabilities and provides a voice service, a data service, and a multi-media service. Various mobile communication standards such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution Advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) of 3GPP2, and 802.16 of the Institute of Electrical and Electronics Engineers (IEEE) have been developed. The mobile communication standards are for supporting a high speed and high quality wireless packet data transmission service.
An LTE system is a system developed to efficiently support high speed wireless packet data transmission. The LTE system may maximize capacity of a wireless system using various wireless access technologies.
The LTE-A system is an evolved wireless system of an LTE system and has improved data transmission performance as compared with the LTE system.
A 3rd Generation (3G) wireless packet data communication system, such as HSDPA, HSUPA, and HRPD, uses an Adaptive Modulation and Coding (AMC) scheme and a channel reply scheduling scheme to improve transmission efficiency. According to the AMC scheme and the channel reply scheduling scheme, a transmitter may receive feedback of partial channel state information and apply a suitable modulation and coding scheme at the most efficiently determined time point.
In a wireless packet data communication system to which the AMC scheme is applied, a transmitter may control an amount of transmitted data according to a channel state. That is, if the channel state is not satisfactory, the transmitter may reduce the amount of the transmitted data to reduce a reception error probability to a desired level. If the channel state is excellent, the transmitter may increase the amount of the transmitted data to efficiently transmit a large amount of information while maintaining a reception error probability to a desired level.
In a wireless packet data communication system to which a channel reply scheduling resource management method is applied, a transmitter selectively provides a service to a user having an excellent channel state among a plurality of users. Accordingly, available system capacity in the channel reply scheduling scheme is increased as compared with a scheme of allocating a channel to one user to serve. Such capacity increase refers to a multi-user diversity gain.
When the AMC scheme is used together with a Multiple Input Multiple Output (MIMO) transmission scheme, it may include a function of determining the number of spatial layers of a transmitted signal or a rank. In this case, the wireless packet data communication system to which the AMC scheme is applied simply considers a code rate and a modulation scheme to determine an optimal data transmission rate and whether to transmit how many layers using the MIMO.
It is known in the art that an increase in capacity is expected in an Orthogonal Frequency Division Multiple Access (OFDMA) scheme as compared with a Code Division Multiple Access (CDMA) scheme. One of various reasons for the increase in capacity in the OFDMA scheme is that a frequency domain scheduling may be performed in a frequency axis. As a capacity gain is acquired through the channel replay scheduling scheme due to a channel change characteristic according to a time, if using a characteristic which a channel is changed according to a frequency, additional capacity gain can be obtained. In recent years, research has been performed on an evolution from the CDMA scheme being a multiple access scheme used in 2nd Generation (2G) and 3G mobile communication systems to an OFDMA scheme being a next generation system. 3GPP and 3GPP2 have worked on standardizing an evolved system using OFDMA.
FIG. 1 is a diagram illustrating a cellular mobile communication system according to the related art.
Referring to FIG. 1, transceiving antennas 130, 140, and 150 are arranged in centers of cells 100, 110, and 120, respectively. In a cellular mobile communication system having a plurality of cells, a certain User Equipment (UE) receives a mobile communication service from one cell selected for a long semi-static period. Here, the mobile communication service is a service using the above various schemes. For example, it is assumed that the cellular mobile communication system is configured by three cells 100, 110, and 120. It is assumed that the first cell 100, the second cell 110, and the third cell 120 provide a mobile communication service to first UEs 101 and 102, a second UE 111, and a third UE 121, respectively.
A distance between the first UE 102 and the antenna 130 is greater than a distance between the first UE 101 and the antenna 130. A signal from a center antenna 150 of another cell 120 interferes with the reception of a signal from the antenna 130 by the first UE 102. Accordingly, data transmission speed between the first UE 102 and the cell 100 becomes relatively low.
When the cells 100, 110, and 120 independently provide a mobile communication service, a Reference Signal (RS) for estimating a channel is transmitted to measure a downlink channel state of each cell. In a case of a 3GPP LTE-A system, a UE measures a channel state between the UE and a base station using a Channel Status Information RS (CSI-RS) which the base station transmits.
FIG. 2 is a diagram illustrating a location of a CSI-RS which a base station transmits to a UE in an LTE-A system according to the related art.
Referring to FIG. 2, a signal with respect to two CSI-RS antenna ports in each location of reference numerals 200 to 219 may be transmitted. That is, the base station transmits two CSI-RSs for measuring the downlink to the UE from a location 200. As shown in FIG. 1, in a case of a cellular mobile communication system having a plurality of cells, locations to which CSI-RS is transmitted may be allocated differently according to cells. For example, in a case of a cell 100 shown in FIG. 1, a CSI-RS may be transmitted from the location 200 in a case of the cell 100 shown in FIG. 1, from the location 205 in a case of the cell 110, and from the location 210 in a case of the cell 120. As described above, a resource for CSI-RS transmission is allocated in different locations with respect to respective cells to prevent CSI-RSs of different cells from causing mutual interference.
A CSI-RS sequence transmitted from locations of CSI-RS antenna ports is defined below in Equation 1 and Equation 2.
                                                        r                              l                ,                                  n                  s                                                      ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            RB                          max              ,              DL                                -          1                                    Equation        ⁢                                  ⁢        1                                          c          init                =                                            2              10                        ·                          (                                                7                  ·                                      (                                                                  n                        s                                            +                      1                                        )                                                  +                l                +                1                            )                        ·                          (                                                2                  ·                                      N                    ID                    cell                                                  +                1                            )                                +                      2            ·                          N              ID              cell                                +                      N            CP                                              Equation        ⁢                                  ⁢        2            
In Equation 1 and Equation 2, l denotes an OFDM symbol order in one slot, and NCP is determined with 0 or 1 according to a length of a Cyclic Prefix (CP) used in a cell.
In the cellular mobile communication system shown in FIG. 1, a UE existing at an edge of a cell interferes with other cells and thereby may cause those other cells to not be able to communicate using a high data transmission rate. That is, in the cellular mobile communication system shown in FIG. 1, a data transmission rate to be provided to UEs existing in a cell is greatly affected according to where is the UE is located in the cell. Accordingly, in the cellular mobile communication system, a UE located relatively close to a cell center may transmit and receive data with a high transmission rate. A UE located in a remote location may not transmit and receive data with a high transmission rate.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.